1. Field of the Invention
This invention relates to an improved process for the trimerization of organic isocyanates. More particularly, it is concerned with the process for the preparation of isocyanurates, polyisocyanurates and cellular foams characterized by isocyanurate linkages.
2. Description of the Prior Art
The prior art methods for the trimerization of organic isocyanates have been disclosed in U.S. Pat. No. 2,993,870 which teaches the use of certain catalytic hexahydrotriazine compounds to facilitate the cyclization reaction. Varied density polyisocyanurate foam articles have also been described as being produced using polyisocyanates and certain hexahydrotriazine catalysts; see for example U.S. Pat. Nos. 3,644,168 and 3,836,427. While these disclosures provide examples of catalysts that are effective in the isocyanate/isocyanurate systems, the activity of the catalysts does not permit facile control of the trimerization reaction. For example, it is well known that small changes in concentrations of these catalysts, within certain concentration ranges exhibit enormous changes in the rate of reaction of the isocyanate group. Further, to effect a sufficiently rapid and economical reaction rate, relatively large quantities of the expensive hexahydrotriazines are necessary. It has heretofore been believed that only substituted triazines could function as useful catalysts for the cyclization/condensation reaction of isocyanates to isocyanurates, thus limiting the selection of useful catalysts.
The present invention is based on the discovery that nitrogen-containing heteocyclic amines other than the prior art hexahydrotriazines are useful in the preparation of isocyanurates and cellular, as well as non-cellular, isocyanurate-containing polymers.
As a further embodiment of the present invention, the method of preparation for these catalytic nitrogen-containing heterocyclic amines is also disclosed. The synthesis of these heterocyclic materials, e.g., the dihydrodioxazines have been revealed in the early prior art as being effected by treating a primary amine with excess formaldehyde. See U.S. Pat. No. 3,527,657. While it was recognized that the products of the reaction are controlled by adjusting the ratios of reactant, i.e., large excesses of formaldehyde result in dioxazines while equimolar reactions yield hexahydrotriazines, the fact that the formation of the triazine, dioxazine, and even the oxadiazine can be readily controlled by adjusting the temperature at which the reaction occurs has not been discovered, and see J. F. Walker Formaldehyde, Reinhold Publishing Corp., 1976, 360 ff.